Light type just don't cut it. Why would we need a new type while Poison only have advantage against grass and Electric only weak to ground? They should balance it first before adding more new type. Type like grass, have too many weaknesses (its even making Celebi and Exeggcutor have 7 weaknesses) and advantage against type like rock, ground which water type also have the same advantage against them in term of offense, which make many people choose water type over grass type. Poison type should have advantage against water type and electric to make poison type to have more use other than spamming Toxic to enemy team.
I don't think Light type is necessary. This is ain't a fantasy world where Light and Darkness always fought each other for world domination.